A coulee is a deep, steep-sided ravine or valley that is typically dry or has only intermittent water flow. The term originates from the Canadian French word coulée, derived from the French verb couler, meaning “to flow.” This reflects the feature’s creation by the powerful action of flowing water, even if the flow is not permanent today.
Defining the Coulee: Characteristics and Scale
Coulees are distinguished from typical river valleys by their structure. Unlike V-shaped valleys carved slowly by streams, a coulee often exhibits a wide, flat bottom and strikingly steep, nearly vertical walls, giving it a distinctive box-shaped cross-section. This structure suggests formation by rapid, massive erosion rather than gradual down-cutting.
Their size generally places them between a small gully or arroyo and a major canyon system. Many coulees are hundreds of feet deep and can stretch for tens of miles. The water found in a coulee today is often a misfit stream, meaning it is too small to have carved the massive channel it occupies. Flow usually only occurs seasonally, triggered by significant snowmelt or heavy rainfall.
These landforms are scars left by ancient, short-lived torrents. The steepness of the walls is maintained by the durable, fractured bedrock into which they are carved. This combination of a flat floor and high walls defines the coulee’s appearance across the landscapes where they are found.
The Geological Origins of Coulees
The formation of coulees resulted from catastrophic erosion, not the slow, steady work of a perennial river. The mechanism involves massive, rapid outflows of glacial meltwater that occurred during the Pleistocene Epoch. These events, often called the Ice Age Floods, were powerful enough to instantaneously alter the landscape.
The floodwaters, moving at speeds estimated up to 65 miles per hour, stripped away topsoil and scoured the underlying bedrock through hydraulic plucking and abrasion. The bedrock in regions where coulees are common, such as the Columbia Plateau, is primarily composed of columnar basalt. This volcanic rock cools into vertical, hexagonal pillars with natural fracture planes.
The immense force of the floodwaters exploited these existing vertical and horizontal fractures in the basalt, allowing large blocks to be plucked away rapidly. This violent erosion created a series of migrating waterfalls, or cataracts, that retreated upstream. As the cataracts eroded the basalt layers, they created the wide, deep, and steep-sided trenches recognized as coulees.
Notable Examples and Geographic Context
The most spectacular concentration of coulees is found in the Channeled Scablands of the Pacific Northwest, primarily in eastern Washington state. This region, a landscape of barren, scoured bedrock, was formed by the repeated catastrophic draining of Glacial Lake Missoula between 18,000 and 15,000 years ago. The largest example is the Grand Coulee.
The Grand Coulee is an ancient riverbed that stretches for approximately 60 miles, featuring walls up to 1,000 feet high. This enormous trench was carved when a lobe of the Cordilleran Ice Sheet blocked the Columbia River, diverting its flow and subsequent floodwaters south across the basalt plateau. The coulee is bisected by Dry Falls, a 3.5-mile-wide cliff that was once the largest waterfall in the world, marking the divide between the upper and lower sections of the coulee.
Another significant example in the Channeled Scablands is Moses Coulee, which runs parallel to the Grand Coulee and served as a major flood pathway. Coulees are also common features in the Canadian Prairies, particularly in southern Alberta and Saskatchewan, where they were carved by meltwater channels during the retreat of the continental ice sheet.